Summary

Some VarHandles created by the Foreign Function & Memory (FFM) API now consistently return false on VarHandle::isAccessModeSupported for access modes that they do not support.

Problem

Some VarHandles returned by FFM APIs, including MemoryLayout::varHandle, arrayElementVarHandle, and ValueLayout::varHandle, may or may not support access modes other than plain get and set, such as getOpaque setVolatile, depending on the byte alignment property of the layout and the path elements passed to construct the handle.

When these access modes are not supported, the implementation methods currently simply throw an UnsupportedOperationException. However, they do not correctly report this lack of support in VarHandle::isAccessModeSupported.

Solution

Update those VarHandle implementations to return false upon isAccessModeSupported query for non-plain access modes when they are not supported by these VarHandle.

Specification

No change. The fact that some access modes are not supported by all var handles created by FFM is already documented here:

https://docs.oracle.com/en/java/javase/23/docs/api/java.base/java/lang/foreign/MemoryLayout.html#access-mode-restrictions

As such, this change is purely behavioral.